The present invention relates to electrooptical switching devices which permit connecting photoemitter input units to photoreceptor output units by optical connectors modifiable under the action of external electric control signals. The application of these devices to telephone switching operations leads to obtaining a telephone exchange in which the interconnection of subscribers' lines is performed without moving electrical contacts. The sphere in question thus consists of the techniques for the spatial switching of beams coming from single-mode or multimode optical fibres, for example.
A prior art device described in the French patent application published under the No. 2 495 873. relates to switches enabling connection by an optical method between at least one of the circuits of a set of input circuits to at least one circuit of a set of output circuits. It consists of illuminating a photosensitive medium with parallel beams emerging from a matrix of input circuits. The beams are then diffracted on strata inscribed in the medium to reach one of the circuits of a matrix of photoreceptor circuits. The strata are inscribed by interference between two beams of which the wavelength differs from that of the input beams; the wavelength of these input beams not being situated within the range of spectral sensitivity of the photosensitive medium. The inscribing of the strata which ensures switching between any two subscribers is performed by means of two beams emerging from a laser. Since the photosensitive material utilized is thick, the wavelength change between the inscription and read-out enforces positioning the recording beams under particular incidences. During the read action, this ensures the deflection of the beam towards the required circuit of the output matrix. The device providing inscription of the grid of strata comprises a double deflection system XY whose task is to pick an optional point in the switching plane and to position the beam with the required incidence on the crystal; this beam thus pivoting around a selected point in the plane of the crystal.
The drawback of this prior art device consists of the difficulty of adjusting the angles of incidence of the recording beams in the photosensitive medium.
The present invention is also based on the angular deflection of the beam by a grid of strata photoinduced in a photosensitive medium. However, it provides a notable simplification of the optical layout for inscription and reading the grids of strata of variable pitch and diffracting the beams under Bragg conditions as compared to the prior art device.
In order to eliminate the disadvantage of the prior art device, the photosensitive medium is actually illuminated in an uniform manner in the present invention, a bidimensional spatial modulator permitting selection of one of the photoemitter circuits.